Proceedings of ICRC 2001: 3473 c Copernicus Gesellschaft 2001

ICRC 2001

THE FIRST OUTCOME ON THE 3-D FEATURE OF FORBUSH DECREASE EVENTS FROMLARGE MUON TELESCOPE OF GRAPES III AT OOTY

The first outcome on the 3-D feature of Forbush decrease events
S.Kawakami (1), K.Fujimoto(3), S.K.Gupta (2), Y.Hayashi (1), N.Ito (1), A.Jain(2), H.Kojima (3), D.K.Mohanty(2), T.Nonaka
fromlarge muon
telescopeofH.Tanaka
GRAPES
IIIT.Toyofukuat Ooty(1), S.Noto (1),K.C.Ravindran(2),K.Satomi(1), K.Sivaprasad (2),(1), S.C.Tonwar (2),(1),K.Viswanathan (2), T.Yoshikoshi (1)S. Kawakami, K. Fujimoto, S. K. Gupta, Y. Hayashi, N. Ito, A. Jain, H. Kojima, D. K. Mohanty, T. Nonaka, S. Noto, K.
C. Ravindran, K. Ssatomi, K. Sivaprasad, H. Tanaka, S. C. Tonwar, T. Toyofuku, K. Viswanathan, and T. Yoshikoshi(1) Osaka City University, OSAKA

Osaka City University, OSAKA(2) Tata Institute of Fundamental Research, MUMBAINagoya
University,NAGOYA(3) NagoyaWomen’s
Women's University,NAGOYAOsaka City University, OSAKA
Abstract
We have observed several Forbush decrease events during 2000 and 2001 with Large Muon Telescope of the GRAPES III shower array atOoty (800g/cm2). Our telescope is rather unique in its very large detection area, total area of 560m2 (16 units of 35m2 each) and rather goodangular resolution, less than 8 degrees. We present the 3-D structure of Forbush Decrease (FD) events and their time profiles in great details.
We found some distinct features in their structures. Some of the CME (Coronal Mass Ejection) from the Solar flares can cause these peculiar
type of FD events. We discuss these results in connection with the energy spectrum of the cosmic rays during the passage of CME directed
toward the Earth.

1advantage to study the global feature of IMF and shock front
associated with the solar flare events.
Another aim is to make a detailed study on time profile of the solar
proton and neutron in the occasion of Grand Level Enhancement(GLE) Kawakami et al (1999) If there would be the rapid change inGLE, it would give the clue to understand the mechanisms ofparticle acceleration in their early stage which would be difficult tostudy by the conventional small size neutron monitor.
We report here the time and space structure of several FDs andwould like to discuss their behavior.

Introduction

Each detector has its own characteristics such as neutron monitors,
scintillation detectors, proportional counters and so on. Our muon
telescope in Ooty GRAPES III experiment has an excellent statistics
because of its large detection area, 560m2. This is a great advantage
for the study on short term variation of both the galactic cosmic raysand solar cosmic rays. Short time feature can be only clarified withlarge area and direction sensitive detector.
One of our main aim is to study the 3-D structure (in time andspace) of Forbush event rather precisely and to understand the effectof solar magnetic field. We are hoping to clarify the nature ofForbush Decrease in connection with Solar flare and Interplanetary
Magnetic Field.(IMF) The satellite observation can perform point
observation very well, but our ground level detector has a great
Correspondenece to kawakami@sci.osaka-cu.ac.jp

2Detectors and Experimental Conditions

The detector element is 6m long proportional counter with cross13474
periodically with cycle of 12 hours. Fig.2 shows a typical exampleof the variation of muon intensity, variation of atmosphericpressure.

Fig.1 Side view of GRAPES III Muon detectors. Figure of 2 modules.

section of 10cm by 10 cm. 58 counters are placed side by side on aconcrete platform and covered with 15cm thick concrete slabs.
Four layers of counters are arranged in crossed configuration toobtain the track and angle of individual muon. Top of the counter
layer thick concrete were put and total thickness of the detector is
550g/cm2 (about 20 r.l.), so the minimum energy of a penetrating
muon is about 1 GeV. Total number of modules for muon detector
is 16. Area of each module is around 35 m2 and its total area is 560
m2 . They have been fully operational since February 1998. Figureof the Muon Telescope is shown in Fig.1.
Omni directional observation has been started for 16 modules fromApril 1998 and the directional measurement of the muons has been
started from April 1999 by one module and now 12 modules (totalarea 420m2) are operational. In principle the angle of individual
muon is recorded in each 10 seconds.
The ambient condition of these muon detectors is quite stable.
Though outside daily temperature varies around 15 degrees for fineweather, temperature of the muon detector deviate only one to twodegrees per day. Maximum temperature variation in a year near thedetectors is within 5 degrees. The atmospheric pressure changes   !"#$ !%$ #      Fig.4. schematic diagram for NE direction

3 Analysis of Forbush Decrease Events


We have categorized the data by the angle of muons, the exampleof combination of angle is shown in Fig.3.
There are 9 categories of muons in total, North, North East, East,South East, South, South West, West, North West and Vertical.
(Fig.4) All the raw data (each one minute interval) have beenchecked up thoroughly. After confirming that there is nomisbehavior due to hardware problems and then summed up all themodules together. We adopted one hour count rate for analysis ofFD through directional observation.
Fig.5 shows typical variation of galactic cosmic rays in FD. Phase
shift of anisotropy is clearly seen through out FD period. It





Fig. 3. groups of the muon direction                

Fig.2 Red: variation of muon intensity
Black: barometric pressure2

3475
complete mixing of cosmic rays for all the direction. It is clearlyseen in Fig.6, there is a peak at around 04:00 for all the direction.
(Indian Standard Time (IST). IST is UT + 5.5 hours.) It shouldmean that the cosmic ray intensity for all the direction varies

indicates that the anisotropy arising from Solar magnetism ismaintained somehow during FD period and only the intensity of all
the direction is disturbed equally. This feature is rather difficult tounderstand, since some shock front must have passed when thecosmic ray decrease sharply. The natural consequence of it would
be the mixing of cosmic rays after the shock front.

&'&&(NE
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E102

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4/11/01

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Fig.5 Example of typical Forbush Decrease

4/12/01

IST (UT + 5.5hr)

4/13/01

Fig.6 Examples of Forbush Decrease where mixing have happened

Contrary to this FD, another type of Forbush decrease shows the3

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modules only for this FD of April 12th, the data of another 9
modules still to be analyzed for this FD. We are hoping to complete
the analysis of remaining data by the conference time and to bringmore concrete conclusion.

simultaneously in this period. So, there could have been somemixing of cosmic rays by the shock front. As a consequence of thismixing, anisotropy is lost for about 1 to 2 hours at least. This might
have happened because of over-up of two successive Coronal
Mass Ejection (CME). We estimate two CME (UT 15:34 9th April
and UT 05:26 10th April) which are responsible for thiscomplicated FD. Another distinct feature of this FD is that it shows
quite different peak intensity between East direction and West. We
have categorized E-W groups and shown in Fig.6 and N-S
combination in Fig.7. As it can be seen in Fig..6, peak intensity ofthe east direction is always bigger than the west during this FD. If
the energy spectrum of the cosmic rays becomes soft during thisFD, cosmic rays from east is easy to penetrate the earth’s magneticfield. It could be the cause of these differences.

3

Acknowledgements. We are grateful to the Ministry of educationof Japan for their partial financial support for this experiment. Theauthors wish to thanks Prof.V.S.Narasimham and other member ofthe TIFR-OCU Proton Decay Collaboration for the loan of
proportional counters used in the muon detector. We are also happy
to acknowledge valuable contributions of G.Paul Francis,K.C.Ravindran, B.Sreenivasa Rao, S.Thirunavukkarasu andV.Viswanathan during the installation, operation and maintenanceof the instrumentation for the GRAPES III array. The help andcooperation of the Radio Astronomy Center for providing sitefacilities for the GRAPES III array are gratefully acknowledged

Summary

It seems we might consider the flows of north-south direction ofGCR to explain the complicated up and down during the FD event.
The energy spectrum of GCR would become softer behind theshock front.
We could not spend enough time to estimate the possible behaviorof the interplanetary magnetic field which must have caused theFD shown above. We hope to be able to analyze them soon
thoroughly. So far we have been able to complete the analysis of 3

References
Kawakami S., 1999, 26th ICRC OG-3.2.24 2754